Characteristics of Group III nitride compound semiconductors (hereinafter referred to as nitride compound semiconductors) such as GaN, AlGaN, GaInN, AlGaInN and AlBGaInN include that they have a larger band gap energy Eg than Group III-V compound semiconductors such as AlGaInAs and AlGaInP, and they are direct transition semiconductors.
Because of the characteristics, attention has been given to the nitride compound semiconductors as materials of semiconductor light emitting devices such as semiconductor laser devices which emit light in a short wavelength range from ultraviolet to green and light emitting diodes (LEDs) capable of emitting light in a wider wavelength range from ultraviolet to red.
These semiconductor light emitting devices are being widely applied as light sources for optical pickups of recording/reproduction of high-density optical disks, light sources of full-color displays and other light emitting devices in environmental fields, medical fields and so on.
Moreover, characteristics of these nitride compound semiconductors include, for example, that the nitride compound semiconductors have a high saturation velocity in a high electric field region, or when they are used as materials of a semiconductor layer and aluminum nitride (AlN) is used as an insulating layer at the formation of a MIS (Metal-Insulator-Semiconductor) structure, the semiconductor layer and the insulating layer can be continuously grown through crystal growth.
Because of the characteristics, attention has been given to the nitride compound semiconductors as materials of high-power high-frequency electronic devices.
Further, the nitride compound semiconductors have the following advantages: (1) they have higher thermal conductivity than GaAs or the like, so they are more suitable for the materials of high-power devices used at a high temperature, compared with GaAs, (2) they have superior chemical stability and higher hardness, so they are device materials with high reliability, and (3) they do not include arsenic (As) in AlGaAs, cadmium (Cd) in ZnCdSe or the like as a material, and do not require a source gas such as arsine (AsH3) or the like, so they are compound semiconductor materials which include no environmental pollutant and no poison and have a low impact on environment.
A problem which arises when a semiconductor device with high reliability is made by the use of the nitride compound semiconductors is that there is no suitable substrate material. In other words, in obtaining a high-quality nitride compound semiconductor layer, the following problems with the nitride compound semiconductors and the substrate material arise.
(1) The nitride compound semiconductors such as GaN, AlGaN and GaInN are strained systems with mutually different lattice constants, so when a film made of a nitride compound semiconductor is formed on a substrate, or when nitride compound semiconductor layers are laminated, strict restrictions on the composition and the thickness of the nitride compound semiconductor layer or the like are imposed to obtain a good-quality crystal film without crystal defect such as crack.
(2) A high-quality substrate lattice-matched to GaN which is a typical nitride compound semiconductor has not been developed yet. For example, a high-quality GaAs substrate lattice-matched to GaAs and GaInP and a high-quality InP substrate lattice-matched to GaInAs have been developed, so it is desired to develop a high-quality GaN substrate in a like manner; however, the GaN substrate is under development.
(3) The substrate materials of the nitride compound semiconductors are required to have resistance to a high crystal growth temperature of approximately 1000° C., and to have resistance to deterioration and corrosion by an atmosphere of ammonia (NH3) which is a material of nitride.
Under the above circumstances, there is no suitable substrate lattice-matched to the nitride compound semiconductors, specifically to GaN at present, so a sapphire (α-Al2O3) substrate is often used as the substrate material.
While the sapphire substrate has an advantage in production control that high-quality 2-inch substrates or 3-inch substrates are stably supplied to markets, it has a technical disadvantage of a large lattice mismatch to GaN of 13%.
For example, even if a buffer layer is disposed between the sapphire substrate and a GaN layer to reduce the lattice mismatch so that a favorable single crystal layer of GaN is epitaxially grown, the defect density reaches, for example, 108 cm−2 to 109 cm−2. Therefore, it is difficult to maintain the operational reliability of the semiconductor device for a long time.
Moreover, the sapphire substrate has the following problems: (1) the sapphire substrate has no cleavage, so it is difficult to stably form a laser facet with high mirror reflectance, (2) sapphire is an insulator, so it is difficult to dispose an electrode on the back side of the substrate as in the case of a GaAs semiconductor laser device, and both of a p-side electrode and an n-side electrode must be disposed on the side of a laminate of the nitride compound semiconductor layers on the substrate, and (3) there is a large difference in the thermal expansion coefficient between the sapphire substrate and the GaN layer, so there are a number of restrictions in a process of forming the device, for example, that when a crystal growth film is thick, a large warp in the substrate occurs even at room temperature, and thereby a crack may occur.
In order to overcome the above problems so as to grow a high-quality nitride compound semiconductor crystal on the sapphire substrate, epitaxial lateral overgrowth (ELO) has been developed.
Referring to FIGS. 10A through 15B, first through fourth examples of conventional configurations of the GaN layer formed by the epitaxial lateral overgrowth will be described below. Incidentally, the configurations in the first through the fourth examples are applicable in the case of forming any other nitride compound semiconductor layer instead of the GaN layer.
The epitaxial lateral overgrowth exploits anisotropy of crystal growth rate that when the GaN layer is epitaxially grown, the growth rate is faster in a <11-20> direction which is a leftward or rightward direction in a paper surface in FIGS. 10A through 15B, and a lateral direction which is a <1-100> direction orthogonal to the paper surface than in a <0001> direction (a direction perpendicular to a c-surface) which is a upward direction in the paper surface. Further, in the first through the fourth examples, the epitaxial lateral overgrowth may be carried out in the <1-100> direction instead of the <11-20> direction which is the lateral direction in the paper surface in FIGS. 10A through 15B. The symbol “-” inside the angle brackets is supposed to be attached above a number at the right of the symbol “-” as shown in FIG. 10C which is described later, however in this specification, the symbol is attached before the number for the sake of convenience.
FIGS. 10A and 10B show the first example. In the configuration of the first example, as shown in FIG. 10A, on a sapphire substrate 10 on which a seed crystal layer 11A is formed, a plurality of masks 12 which is made of an insulating film of silicon oxide (SiO2), silicon nitride (SiN) or the like, or a multilayer film including a plurality of the insulating films is formed in stripes, and then as shown in FIG. 10B, a GaN layer 15 which is a crystal layer is laterally grown on the seed crystal layer 11A by ELO so as to cover the masks 12.
Further, FIG. 10C shows that in FIGS. 10A and 10B, the upward direction in the paper surface, the lateral direction in the paper surface and the direction orthogonal to the paper surface correspond to the <0001> direction (a direction perpendicular to the c-surface), the <11-20> direction and the <1-100> direction, respectively. The same is true in FIGS. 1 through 8E and FIGS. 11A through 17B.
FIGS. 11A and 11B shows the second example. In the second example, after the seed crystal layer 11A is formed all over the sapphire substrate 10, for example, a SiO2 film is formed on the seed crystal layer 11A so as to form the mask 12 in a stripe shape, and then as shown in FIG. 11A, by the use of the mask 12, the seed crystal layer 11A is selectively etched until the sapphire substrate 10 is exposed, thereby a seed crystal portion 11 is formed. At this time, a top portion of the sapphire substrate 10 is selectively etched by the use of the mask 12 so as to form a gap 31.
Next, as shown in FIG. 11B, the GaN layer 15 is grown from side surfaces of the seed crystal portion 11 by the epitaxial lateral overgrowth. At this time, the gap 31 is formed between the sapphire substrate 10 and a lateral growth layer, so the growth is smoothly carried out.
FIGS. 12A and 12B show a modification of the second example. In this case, the seed crystal layer 11A with a relatively large film thickness is formed all over the sapphire substrate 10, and then as shown in FIG. 12A, an insulating film, for example, a SiO2 film is formed on the seed crystal layer 11A, and is patterned so as to form the mask 12 in a stripe shape. By the use of the mask 12, the seed crystal layer 11A is etched until the sapphire substrate 10 is exposed, thereby the seed crystal portion 11 is formed. Next, while the mask 12 remains on the seed crystal portion 11, as shown in FIG. 12B, the GaN layer 15 is grown from the side surfaces of the seed crystal portion 11 by the epitaxial lateral growth.
FIGS. 13A and 13B show the third example. As shown in FIG. 13A, in the third example, a configuration equivalent to the configuration of the second example shown in FIG. 11A without the mask 12 is formed.
Then, as shown in FIG. 13B, the GaN layer 15 is grown from the side surfaces, etc. of the seed crystal portion 11 by the epitaxial lateral growth.
FIGS. 14A and 14B show a modification of the third example. In this case, as shown in FIG. 14A, a configuration equivalent to the modification of the second example shown in FIG. 12A without the mask 12 is formed.
Then, as shown in FIG. 14B, the GaN layer 15 is grown from the side surfaces, etc. of the seed crystal portion 11 by the epitaxial lateral growth.
Further, in the third example and the modification thereof, after the seed crystal layer 11A is etched by the use of the mask 12 so as to form the seed crystal portion 11, the mask 12 is removed, and then the GaN layer 15 is laterally grown.
FIGS. 15A and 15B show the fourth example. In the fourth example, the seed crystal layer 11A with a relatively large film thickness is formed all over the sapphire substrate 10, and then as shown in FIG. 15A, an top portion of the seed crystal layer 11A is selectively etched so as to form a projected portion 13 in a stripe shape, thereby the seed crystal portion 11 is formed. After that, the mask 12 is formed on the seed crystal layer 11A except for the top surface of the seed crystal portion 11 and its surroundings. Next, as shown in FIG. 15B, the GaN layer 15 is grown from the top surface and its surroundings of the seed crystal portion 11 by the epitaxial lateral overgrowth.
In the first through the fourth examples and the modifications which are described above, as shown in FIGS. 10B, 11B, 12B, 13B, 14B and 15B, the GaN layer 15 includes a lateral growth region 21 and a high defect density region 22 or only the lateral growth region 21. For example, the lateral growth region 21 is an excellent crystal growth region, but on the other hand, in the high defect density region 22, due to lattice mismatch between the sapphire substrate 10 and GaN, or the like, a crystal defect are introduced from the seed crystal portion 11 with a high crystal defect density of 108/cm2 or over or the seed crystal layer 11A.
More specifically, the lateral growth region 21 is a region formed only through laterally growing GaN, so no crystal defect (dislocation) or a small number of crystal defects are introduced into the region from the seed crystal portion 11 or the seed crystal layer 11A. Therefore, the region is a high-quality GaN layer, that is, a low defect density region.
On the other hand, the high defect density region 22 is a high defect density region into which the crystal defects are introduced from the seed crystal portion 11 or the seed crystal layer 11A. Further, even in the lateral growth region 21, a region where the lateral growth regions 22 are met each other, that is, a region in the vicinity of a meeting portion 32 indicated by a solid line is a high defect density region.
The crystal defect includes screw dislocation, mixed dislocation and edge dislocation, and the defects which occurs in the high defect density region 22 or the region in the vicinity of the meeting portion 32 are mainly the screw dislocation and the mixed dislocation, so a dislocation extending substantially in a c-axis direction (upward in the drawings) is large.
Moreover, in the second example and the modification thereof, as shown in FIGS. 11B and 12B, the GaN layer 15 includes only the lateral growth region 21, which is the low defect density region, although the meeting portion 32 which is the high defect density region is formed. Further, as indicated by a line, a dislocation 33 often occurs in the vicinity of an end of the mask 12.
In the third example and the modification thereof, as shown in FIGS. 13B and 14B, the lateral growth region 21 which is the low defect density region and the high defect density region 22 which is a regrowth layer directly on the seed crystal portion 11 are comprised.
A method of reducing the high defect density region 22 by carrying out first lateral growth, and then carrying out second lateral growth in a position shifted a half cycle of a pattern with a projection and a depression from a position where the first lateral growth is carried out has been proposed. However, defects or the like in the meeting portion still remain, so a high-quality GaN layer cannot be formed all over the substrate.
Thus, even in the first through the fourth examples and combinations of the examples, it is difficult to obtain a substrate with a low defect density as a whole.
It is considered that when the thickness of a crystal growth film including a device portion such as the semiconductor laser device is nearly equal to the cycle of the crystal portion or the mask, a defect distribution during growth in a substantially lateral direction is reflected to the uppermost surface of a laminate including the device portion, so crystal defects occur in the device portion.
Therefore, in order to form the nitride semiconductor device having an excellent GaN layer without defect, it is required to form the semiconductor device on a region not including the high defect density region or a high defect density region in the vicinity of the meeting portion, that is, the lateral growth region.
As an example of the nitride semiconductor device, the configuration of a GaN semiconductor laser device will be described below referring to FIG. 16. The GaN semiconductor laser device comprises the GaN layer 15, and a laminate including an n-side contact layer 41, an n-side cladding layer 42, an active layer 43, a p-side cladding layer 44 and a p-side contact layer 45, all of which are made of a nitride compound semiconductor, in this order on the sapphire substrate 10 with the seed crystal portion 11 in between.
In the laminate, an upper portion of the p-side cladding layer 44 and the p-side contact layer 45 are formed as a laser stripe portion 50 extending in a ridge stripe shape in one direction. As the laser stripe portion 50 is a main device component which emits light when an injected current passes therethrough, the laser stripe portion 50 is aligned so as to be located on the lateral growth region 21 away from the high defect density region 22.
An upper portion of the n-side contact layer 41, the n-side cladding layer 42, the active layer 43 and a bottom portion of the p-side cladding layer 44 are formed as a mesa portion extending in the same direction as the direction in which the laser stripe portion 50 extends.
Further, a protective film 49 made of a SiN film is formed all over the surface, and through apertures disposed in the protective film 49, a p-side electrode 46 and a p-side contact electrode 46A are formed on the p-side contact layer 45 and an n-side electrode 47 and an n-side contact electrode 47A are formed on the n-side contact layer 41.
In order to design and form the semiconductor laser device with excellent laser properties and high reliability, it is important to form the laser stripe portion 50 on the lateral growth region 21, not on the high defect density region 22 and the meeting portion 32.
Referring to the third example and the modification thereof as examples and FIGS. 13B and 14B, a relationship between a width WL of the lateral growth region 21 and a pitch WP (the sum of a width of the seed crystal portion 11 and a width of a region between adjacent seed crystal portions 11) of the seed crystal portion 11 will be described below.
Assuming that the pitch WP is 15 μm and a width WO of the seed crystal portion 11 is 3 μm, the high defect density region 22 directly on the seed crystal portion 11 has a low quality because the crystal defects in the seed crystal portion 11 are introduced into the high defect density region 22, however, the other region with a width of WP−WO=15−3=12 μm, that is, the lateral growth region 21 is the low defect density region, that is, a high-quality region.
However, in fact, as shown in FIG. 13B or 14B, the GaN layer 15 is formed through laterally growing GaN crystals from both side surfaces of the seed crystal portion 11, so in the meeting portion 32, the crystals are not fully matched, thereby resulting in the occurrence of defects. Therefore, the width WL of the lateral growth region having a continuous low defect density is one-half of the width of WP−WO, that is, WL=6 μm.
Next, referring to FIG. 16, the alignment of the laser stripe portion 50 of the GaN semiconductor laser will be described below. In order to obtain the GaN semiconductor laser device with high reliability, as described above, the overall width of the laser stripe portion 50 is required to be arranged on the lateral growth region 21.
For example, assuming that a width WT of the laser stripe portion 50 is 2 μm and the width WL is 6 μm, and the width of the meeting portion 32 is not taken into account, in order to arrange the laser stripe portion 50 within WL=6 μm, the alignment accuracy is required to be ±2 μm.
Further, when the cycle of the laser stripe portion 50 is designed to be an integral multiple of a cycle of the seed crystal portion 11, a periodic configuration can be formed on the whole surface of a wafer.
Moreover, in the configurations shown in FIGS. 10A through 15B, a length of a resonator of a laser in a depth direction of the paper surface is, for example, 200 μm to 1000 μm or over, so compared with the width WT of the laser stripe portion 50, it is sufficiently long so that the same sectional shape can be formed. Therefore, there is no problem in forming the configuration in this direction.
For example, when the substrate material and the crystal film are both transparent, even if a reference position is confirmed by the buried mask 12, the gap 31 or the like so as to align the laser stripe portion 50, in fact, it is often difficult to accurately align the laser stripe portion 50 directly on the lateral growth region 21 not including the meeting portion 32 with high controllability and the above alignment accuracy of ±2 μm, because of the following restrictions.
The restrictions include: (1) the high defect density region 22 directly on the seed crystal portion 11 expands in the thickness direction (in the upward direction in the drawings), (2) the spreading width of the meeting portion 32 is not zero but, for example, approximately 0.5 μm to 1 μm, (3) it is technically difficult to expand the lateral growth region 21, and the width WL of the lateral growth region 21 has an upper limit because of crystal quality control of the lateral growth, (4) the width WO of the seed crystal portion 11 has a lower limit of, for example, 1 μm to 2 μm, and (5) in order to align the laser stripe portion 50 by seeing through the substrate, the alignment accuracy is approximately 1 μm to 2 μm.
Because of these restrictions, for example, in the third example (refer to FIG. 13B), WP=WO+2×WL, WP>2×WL, that is, the width WL of the lateral growth region 21 is designed to be ½ or less of the pitch WP of the seed crystal portion 11 at the maximum.
Further, the value of the pitch WP cannot be freely increased, as described in the above restriction (3) on the crystal growth. For example, the upper limit of the pitch WP is approximately 10 μm, so there is a restriction on the upper limit of the width WL.
Thus, in spite of the fact that the sum of the widths WL of the lateral growth region 21 is 2×WL, there is the meeting portion 32 with poor crystal quality because the adjacent lateral growth regions 21 are met each other, so in substance, a region of only half of the widths 2×WL can be used to arrange the overall width of the laser stripe portion 50.
Moreover, for example, in MOCVD (Metal Organic Chemical Vapor Deposition), the epitaxial growth is carried out while keeping growth conditions in equilibrium, so even if the flow direction of, for example, a source gas crosses the seed crystal portion 11, the meeting portion 32 is formed in a position near the center between the adjacent seed crystal portions 11.
In the above description, although problems are described referring to the GaN layer as an example, they are universal problems when a laminate of the nitride compound semiconductor layers is formed.
In view of the foregoing, it is an object to provide a nitride semiconductor device having higher reliability and capable of increasing the flexibility in device design and a manufacturing margin, and a method of manufacturing the same.